With the improved three port circulators such as shown in U.S. Pat. No. 4,801,901, it is difficult to achieve optimized performance at the same frequency of all three parameters: insertion loss, isolation, and return loss. Insertion loss is the decrease in the signal level between the first input port, and the second, through port. All the signals from the input port should go to the second port and none should go to the third port. Such total separation is practically unobtainable; there are some losses between the first and second ports and some signals leak to the third port. The leakage to the third port is a measure of isolation. In such devices isolation is very good but it is not total. In addition ideally the incoming signal should be absorbed entirely and without reflection. But that is not practically achievable and this is measured as return loss.
The difficulty in maximizing performance of such devices is increased by the fact that each of those characteristics--insertion loss, reflection loss and isolation--tend to be optimized at a different frequency. Thus maximizing performance with respect to one reduces performance with respect to the other two. This non-symmetrical conduct with respect to the center frequency compels a compromise which results in less than optimal performance.